Pyrylium salts are of great use, especially as intermediates in organic synthesis for materials such as phenols, vitamin precursors such as vitamin E, pyridines and anilines.
Applications of pyrylium salts have been recently reviewed by A. T. Balaban et al in Advances in Heterocyclic Chemistry, Suppl. 2, Academic Press, N.Y., 1982.
The conventional method for preparing pyrylium salts having substituents in the 2 and 6 positions comprises diacylating alkenes or alkene precursors such as tertiary alkyl halides or secondary or tertiary alkyl alcohols in the presence of Lewis acids or strong protonic acids such as perchloric acid or trifluoromethane sulfonic acid with the aid of acid anhydrides or acid halides as acylating agents.
In A. T. Balaban and C. D. Nenitzescu (Journal of Chemical Society, 1961, page 3553) the diacylation of 2-methyl-2-butene with acetic anhydride or acetyl chloride and a strong acid such as perchloric acid or sulfuric acid to achieve a mixture of 2,3,4,6-tetramethylpyrylium salt and 4-ethyl-2,6-dimethylpyrylium salt is described. The proportion of the more substituted isomer (2,3,4,6-tetramethylpyrylium salt) was only about 80%.
In R. D. Howells and J. D. McCown (Chemical Reviews, February 1977, page 69) pyrylium salts were prepared using trifluoromethane sulfonic acid as an acylation catalyst.
In the above prior art processes the selectivity of obtaining the most substituted isomer is undesirably low. A mixture of isomers is obtained, said mixture being difficult to purify. In the above processes wherein perchloric acid is used as an acid catalyst, pyrylium perchlorates are formed. These materials are particularly unstable and are very difficult to store and handle.
There has been a need to obtain a process having a high specificity for preparing the most highly substituted pyrylium salts in high yields. The need of such a process is particularly noted in the synthesis of substituted phenols, pyridines and anilines with substitution patterns not otherwise obtainable.